1. Field of the Invention
This invention relates to an electrophotographic printer, and more particularly to an electrophotographic printer which employs an improved image processing technique of printing an image produced by a controller with a resolution increased to 1.5 times.
2. Description of the Related Art
An electrophotographic printer of the type mentioned is disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 1-171946. In the prior art electrophotographic printer, the frequency of a reference clock signal for write data is varied to adjust the size of an image of the write data.
FIG. 8 shows a block diagram of an electrophotographic printer of the type just mentioned. Referring to FIG. 8, an image picture signal from a controller 100 is temporarily stored into a shift register 102, from which it is read out later in accordance with a period of pulses from a clock frequency division circuit 103 and supplied to an engine 101. When the ratio between the image resolution of the controller 100 and the printing resolution of the engine 101 is 2:3, in order to lower the transfer frequency for the picture element signal in regard to a main scanning direction which is a scanning direction of a beam of light to 2/3 times, the clock signal from the controller 100 is frequency divided to 2/3 times by the clock frequency division circuit 103, and with regard to a sub scanning direction perpendicular to the main scanning direction, same data as those of the last line are outputted at the rate of one line to two lines.
Further, for the sub scanning direction, also another method for producing output data has been proposed wherein picture elements of a line to be inserted are produced by logical ORing or logical ANDing of picture elements of the last line and the following next line.
The prior art electrophotographic printer is disadvantageous in the following points.
In order to enlarge image data in the sub scanning direction to 1.5 times, an artificial line is inserted at the rate of one line to two lines. In order to produce picture elements of the line to be inserted, one of a method of duplicating picture elements of the last line (the method will be hereinafter referred to as "duplication method"), another method of logically ORing picture elements of the last line and the following next line (the method will be hereinafter referred to as "logical ORing method") and a further method of logically ANDing picture elements of the last line and the following next line (the method will be hereinafter referred to as "logical ANDing method") is employed.
However, whichever one of those methods is employed, since picture elements to be inserted are black or white in units of a picture element, the enlarged image exhibits striking irregularity in regard to line width or cyclical repetitions of a pattern.
FIGS. 6(a) to 6(d) and 7(a) to 7(d) show original images and enlarged images of them. In particular, FIGS. 6(a) and 7(a) show original images; FIGS. 6(b) and 7(b) show enlarged images obtained by enlargement of the original images by the duplication method; FIGS. 6(c) and 7(c) show enlarged images obtained by the logical ORing method; and FIGS. 6(d) and 7(d) show enlarged images obtained by the logical ANDing method. As can be seen from those figures, those methods are peculiarly disadvantageous in that the duplication method is inferior in reproducibility of a thin line; according to the logical ORing method, an enlarged image exhibits an increase in concentration, and a character or an image is liable to collapse; and according to the logical ANDing method, an enlarged image exhibits a decrease in concentration, and the continuity of a line is liable to be damaged.